1. Field of the Invention
The invention relates to a mold element for injection molding plastic material parts. The invention also relates to a mold including the mold element. The invention further relates to a method of injection molding a plastic material part and a part produced by the method, notably a part for a lighting and/or signalling device.
2. Description of the Related Art
The fabrication of plastic parts by injection molding is well known in itself and routinely practiced. Injection molding, also known as plastic injection, is a method of using thermoplastics. Most thermoplastic parts are fabricated with plastic injection presses: the plastic material is softened and then injected into a mold, and then cooled. Injection molding is a fabrication technique for mass producing parts. It concerns above all plastic materials and elastomers (rubbers), but also diverse metals and alloys with a relatively low melting point: alloys of aluminum, zinc (Zamak), brass or alloys of magnesium in particular transformed by a thixomolding method.
FIG. 1 is a sectional view of a known mold 2. It essentially comprises two half-molds 4 and 6 disposed one against the other and forming a volume 8 designed to be filled with plastic material to form the part. The half-mold 6 includes an injection nozzle 10 fed with plastic material via the passage 12 and actuated by a driving device 14.
The area where the plastic material enters the volume 8 formed by the mold 2 is shown in more detail in the lower portion of the figure corresponding to an enlargement of the encircled portion of the mold 2. The injection nozzle 10 feeds the volume 8 at a low point so that the plastic material first fills the lower portion of the part and then fills the upper portion. The plastic material 18 is represented by a darker area in the FIG. 1 enlargement. The volume 8 includes a cavity 16 for forming a projection from the rest of the part to be formed. During injection of the plastic material, the latter material will partially fill the cavity 16 without filling it completely because of the accumulation of air in the far end of the cavity. The incomplete filling of the cavity is clearly visible in FIG. 1.
FIG. 2 shows in more detail the phenomenon of formation of a defect at the level of the cavity 16 of the volume 8 from FIG. 1. The plastic material 18 flowing into the volume partially fills the latter, the upper level being illustrated by a surface rounded in the manner of convex meniscus. The incomplete formation of the projecting portion 20 of the part may be seen. This portion 20 has a clearance angle greater than or equal to 1° relative to the internal surface of the cavity 16. The end face 22 of the projecting portion 20 moreover has a surface that is not plane and not regular. When the plastic material 18 flowed into the cavity, air escaped and was drawn downstream. As a function of diverse rheological parameters, notably the viscosity of the plastic material and the speed of the flow, some of this air may remain trapped in an area 23 disposed downstream of the entry of the cavity 16. As a function of diverse rheological parameters, when it passes in front of the cavity, the flow may also be disrupted by sudden changes of thickness, also generating diverse defects in the part. The result of these phenomena is defects of different types in the finished part.
What is needed, therefore, is a system and method that overcomes one or more of the aforementioned problems.